[lambertsimnel]

I think there's a third possibly: namely that Congress has somehow allowed the FBI to make such rules, but could reclaim control in the future.

As I understand it, a lot of things work like this in some unitary (as opposed to federal), parliamentary (as opposed to presidential) democracies. In at least some such countries all governmental power ultimately depends on the elected legislature, but it can be delegated to other bodies (including to the executive and to local government) as the legislature chooses. The legislature can reclaim its power, too, even from directly-elected local authorities.